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Entanglement and swap of quantum states in two qubits

Suppose that two distant parties Alice and Bob share an entangled state $ρ_{AB}$, and they want to exchange the subsystems of $ρ_{AB}$ by local operations and classical communication (LOCC). In general, this LOCC task (i.e. the LOCC transformation of $ρ_{AB} \to Vρ_{AB} V$ with $V$ being a swap operator) is impossible deterministically, but becomes possible probabilistically. In this paper, we study how the optimal probability is related to the amount of entanglement in the framework of positive partial transposed (PPT) operations, and numerically show a remarkable class of states whose probability is the smallest among every state in two quantum bits.